Closeout panels can present problems for manufacturers, in that panels may attach to a substructure without access to the backside of the panel. In the past, these panels have been bolted to the substructure or attached using blind fasteners, such as pull rivets. These methods require expensive and time-consuming drilling and fastening operations and may weaken the structure. More recently, these panels have been co-bonded or secondarily bonded using resin or a thin layer of adhesive.
Typically, laminating resins are used as the matrix material in woven textiles, this also being true for woven preforms used to connect components made of composites or other materials. An example of a commonly used laminating resin is 977-3, available from Cytec Industries, Inc., of West Paterson, N.J. The laminating resin is infused into a textile product and is cured to form a polymer matrix in the finished composite component. When assembling a typical joint using a preform, the preform may be co-cured along with uncured composite components or the components may be cured prior to assembly using an uncured preform. Because of the inferior bonding characteristics of laminating resins, a thin layer of adhesive is often placed between the preform and the components. Generally, an adhesive film is used, which is expensive and adds to fabrication time.
To achieve proper bonding when using a thin layer of adhesive, such as an adhesive film, between pre-cured components, special attention must be paid to the interface at the adhesive layer. This bond line is critical, and, where two surfaces are brought together, the distance between the surfaces must be within a critical tolerance to ensure a proper bonding layer. The thickness of the adhesives is usually about 0.015″ thick with a bond layer tolerance of +/−0.005″. Methods for ensuring proper bonding may include tools, such as molds or vacuum bags, but particular applications may prevent the use of tools due to the inaccessibility of one or both sides of the joint. An example of this type of application is a closeout panel, such as the skin of a wing being bonded to an internal spar.
Z-pins have been used in joints connecting two composite, laminate components in the prior art. For example, U.S. Pat. Nos. 5,863,635, 5,968,639, and 5,980,665 to Childress discloses inserting z-pins into a first composite component to form stubble at a bonding face, then curing the first component. An uncured second component is then bonded to the first component with the stubble extending into and among the fibers of the second component and through the bond line.
As shown in FIG. 1 and in the '635, '639, and '665 patents, an additional prior-art method includes inserting a padup strip 11 between two cured components 13, 15. Components 13, 15 are generally formed of plies of woven or unidirectional fibers and a resin matrix and are cured with a Z-pin stubble extending from surfaces 17, 19. Padup strip 11, which is typically formed of the same materials as components 13, 15 or formed of a pure adhesive material without fiber reinforcement, is uncured during assembly. Components are assembled with padup strip 11 between surfaces 17, 19, the z-pin stubble fields extending into padup strip 11. The resin in padup strip 11 is then cured to co-bond the components 13, 15 to padup strip 11.
An alternative method of assembly using z-pins is disclosed in U.S. Pat. Nos. 5,876,540, 5,876,832, 5,935,698 to Pannell and is shown in FIG. 2. A pre-cured strip 21 is formed of a plurality of plies of fibers and a resin matrix, a plurality of z-pins 23 extending from opposite sides of strip 21. Components 25, 27 are also formed of composites and may be cured or partially cured. To assemble partially cured components 25, 27, strip 21 is positioned between components 25, 27, then z-pins 23 are inserted into adjacent surfaces 29, 31. The resin in components 25, 27 is cured to co-bond surfaces 29, 31 and to retain z-pins 23 within components 25, 27. Alternatively, if components 25, 27 are pre-cured, padup strips 33 are used between strip 21 and surfaces 29, 31. Padup strips 33, like padup strip 11 in FIG. 1, are typically formed of the same materials as components 25, 27 or formed of a pure adhesive material without fiber reinforcement.
A need exists for an improved method that reduces the steps in assembly and provides for a strong joint when joining components using a woven preform. A further need exists for a method of joining components in a structural joint that provides for a larger dimensional tolerance between components when using an adhesive at the bond line.